Chapter 47 Animal Development
Overview: A Body-Building Plan for Animals
From egg to organism, an animal’s form develops gradually.
• The question of how a zygote becomes an animal has been asked for
• As recently as the 18th century, the prevailing idea was preformation,
the notion that an egg or sperm contains an embryo that is a
preformed miniature adult.
• The competing theory is epigenesis, proposed 2,000 years earlier by
• According to epigenesis, the form of an animal emerges from a
relatively formless egg.
• As microscopy improved in the 19th century, biologists could see that
embryos took shape in a series of progressive steps.
• Epigenesis displaced preformation as the favored explanation
• Both preformation and epigenesis have some legitimacy.
• Although the embryo’s form emerges gradually as it develops,
aspects of the developmental plan are already in place in the
eggs of many species.
• An organism’s development is primarily determined by the
genome of the zygote and also by differences that arise
between early embryonic cells.
• These differences set the stage for the expression of different
genes in different cells.
• In some species, early embryonic cells become different because of
the uneven distribution within the unfertilized egg of maternal
substances called cytoplasmic determinants. • These substances affect development of the cells that inherit
them during the early mitotic divisions of the embryo.
• In other species, the differences between cells are due to their
location in the developing embryo.
• Most species establish differences between early embryonic cells by
a combination of these two mechanisms.
• As development continues, selective gene expression leads to cell
differentiation, the specialization of cells in structure and function.
• Along with cell division and differentiation, development involves
morphogenesis, the process by which an animal takes shape.
Concept 47.1 After fertilization, embryonic development proceeds
through cleavage, gastrulation, and organogenesis
Fertilization activates the egg and brings together the nuclei of sperm
• The gametes (egg and sperm) are both highly specialized cell types.
• Fertilization combines haploid sets of chromosomes from two
individuals into a single diploid cell, the zygote.
• Another key function of fertilization is activation of the egg.
• Contact of the sperm with the egg’s surface initiates metabolic
reactions within the egg that trigger the onset of embryonic
• Sea urchin fertilization has been extensively studied.
• Sea urchin egg and sperm encounter each other after the animals
release their gametes into seawater.
• The jelly coat of the egg attracts the sperm, which swims
toward the egg.
• When the head of the sperm comes into contact with the jelly coat,
the acrosomal reaction is triggered, and the acrosome, a specialized
vesicle at the tip of the sperm, discharges its contents by exocytosis. • Hydrolytic enzymes enable the acrosomal process to penetrate
the egg’s jelly coat.
• The tip of the acrosomal process adheres to special receptor
proteins on the egg’s surface.
• These receptors extend through the vitelline layer, just external
to the egg’s plasma membrane.
• This lock-and-key recognition ensures that eggs will be
fertilized only by sperm of the same species.
• The sperm and egg plasma membranes fuse, and the sperm nucleus
enters the egg’s cytoplasm.
• Na+ channels in the egg’s plasma membrane open.
• Na+ flows into the egg, and the membrane depolarizes,
changing the membrane potential of the egg.
• Such depolarization is common in animals.
• Occurring within 1–3 seconds after the sperm binds to the egg,
depolarization prevents additional sperm from fusing with the egg’s
• This fast block to polyspermy prevents polyspermy, the
fertilization of the egg by multiple sperm.
• Fusion of egg and sperm plasma membranes triggers a signal-
• Ca2+ from the egg’s endoplasmic reticulum is released into the
cytosol and propagates as a wave across the fertilized egg.
• High concentrations of Ca2+ cause cortical granules to fuse with the
plasma membrane and release their contents into the perivitelline
space, the space between the plasma membrane and the vitelline
• The vitelline layer separates from the plasma membrane.
• An osmotic gradient draws water into the perivitelline space,
swelling it and pushing it away from the plasma membrane. • The vitelline layer hardens into a fertilization envelope, which
resists the entry of additional sperm.
• The fertilization envelope and other changes in the egg’s
surface function together as a long-term slow block to
• The plasma membrane returns to normal, and the fast block to
polyspermy no longer functions.
• High concentrations of Ca2+ in the egg stimulate an increase in the
rates of cellular respiration and protein synthesis, activating the egg.
• Unfertilized eggs can be activated artificially by the injection of Ca2+
or by a variety of mildly injurious treatments, such as temperature
• It is even possible to activate an egg that has had its nucleus
• Evidently, proteins and mRNAs present in the cytoplasm of the
unfertilized egg are sufficient for egg activation.
• As the metabolism of the activated egg increases, the sperm nucleus
swells and merges with the egg nucleus, creating the diploid nucleus
of the zygote.
• DNA synthesis begins and the first cell division occurs about 90
minutes after fertilization.
• Fertilization in terrestrial animals, including mammals, is generally
• Secretions in the mammalian female reproductive tract alter certain
molecules on the surface of sperm cells and increase sperm motility.
• The mammalian egg is surrounded by follicle cells also released
• A sperm must migrate through a layer of follicle cells before it
reaches the zona pellucida, the extracellular matrix of the egg. • Binding of the sperm cell to a receptor on the zona pellucida
induces an acrosomal reaction similar to that seen in the sea
• Enzymes from the acrosome enable the sperm cell to penetrate the
zona pellucida and bind to the egg’s plasma membrane.
• The binding of the sperm cell to the egg triggers changes within
the egg, leading to a cortical reaction, the release of enzymes
from cortical granules to the outside via exocytosis.
• The released enzymes catalyze alteration of the zona
pellucida, which functions as a slow block to polyspermy.
• The entire sperm, tail and all, enters the egg.
• A centrosome forms around the centriole that acted as
the basal body of the sperm’s flagellum.
• This centrosome duplicates to form the two centrosomes
of the zygote.
• These will generate the mitotic spindle for the first cell
• The envelopes of both egg and sperm nuclei disperse.
• The chromosomes from the two gametes share a common
spindle apparatus during the first mitotic division of the zygote.
• Only after the first division, as diploid nuclei form in the two
daughter cells, do the chromosomes from the two parents come
together in a common nucleus.
• Fertilization is much slower in mammals than in the sea urchin.
• The first cell division occurs 12–36 hours after sperm binding in
Cleavage partitions the zygote into many smaller cells.
• A succession of rapid cell divisions called cleavage follows
fertilization. • During this period, cells go through the S (DNA synthesis) and
M (mitosis) phases of the cell cycle but may skip the G1 and G2
• As a result, little or no protein synthesis occurs.
• The first five to seven divisions form a cluster of cells known as the
• A fluid-filled cavity called the blastocoel forms within the morula,
which becomes a hollow ball of cells called the blastula.
• The zygote is partitioned into many smaller cells called
• Each blastomere contains different regions of the
undivided cytoplasm and, thus, may contain different
• Most animals have both eggs and zygotes with a definite polarity.
• Thus, the planes of division follow a specific pattern relative to
the poles of the zygote.
• Polarity is defined by the heterogeneous distribution of
substances such as mRNA, proteins, and yolk.
• Yolk is most concentrated at the vegetal pole and least
concentrated at the animal pole.
• In amphibians, a rearrangement of the egg cytoplasm occurs at the
time of fertilization.
• The plasma membrane and cortex rotate toward the point of
• The gray crescent is exposed, marking the dorsal surface
of the embryo.
• Molecules in the vegetal cortex are now able to interact with
inner cytoplasmic molecules in the animal hemisphere, leading
to the formation of cytoplasmic determinants that will later
initiate development of dorsal structures. • Thus, cortical rotation establishes the dorsal-ventral (back-
belly) axis of the zygote.
• In frogs, the first two cleavages are vertical and result in four
blastomeres of equal size.
• The third division is horizontal, producing an eight-celled
embryo with two tiers of four cells.
• The unequal division of yolk displaces the mitotic apparatus
and cytokinesis toward the animal end of the dividing cells in
• As a result, animal blastomeres are smaller than those in
the vegetal hemisphere.
• Continued cleavage produces a morula and then a blastula.
• Because of unequal cell division, the blastocoel is located in the
• Animals with less yolk (such as the sea urchin) also have an animal-
• However, the blastomeres are similar in size, and the
blastocoel is centrally located.
• Yolk has its most pronounced effect on cleavage in the eggs of
reptiles, many fishes, and insects.
• The yolk of a chicken egg is actually an egg cell, swollen with
• Cleavage of a fertilized bird’s egg is restricted to a small disk of yolk-
free cytoplasm, while yolk remains uncleaved.
• The incomplete division of a yolk-rich egg is meroblastic
• It contrasts with holoblastic cleavage, the complete cleavage of
eggs with little or moderate yolk.
• Early cleavage in a bird embryo produces a cap of cells called the
blastoderm, which rests on undivided egg yolk. • The blastomeres sort into upper and lower layers, the epiblast
and the hypoblast.
• The cavity between these two layers is the avian version of the
• This stage is the avian equivalent of the blastula.
• In insects, the zygote’s nucleus is located within the mass of yolk.
• Cleavage begins with the nucleus undergoing mitotic divisions,
unaccompanied by cytokinesis.
• These mitotic divisions produce several hundred nuclei, which
migrate to the outer edge of the embryo.
• After several more rounds of mitosis, plasma membranes form
around each nucleus, and the embryo, the equivalent of a
blastula, consists of a single layer of 6,000 cells surrounding a
mass of yolk.
Gastrulation rearranges the blastula to form a three-layered embryo
with a primitive gut.
• Gastrulation rearranges the embryo into a triploblastic gastrula.
• The embryonic germ layers are the ectoderm, the outer layer of
the gastrula; the mesoderm, which fills the space between
ectoderm and endoderm; and the endoderm, which lines the
• Sea urchin gastrulation begins at the vegetal pole where individual
cells detach from the blastula wall and enter the blastocoel as
migratory mesenchyme cells.
• The remaining cells flatten to form a vegetal plate that buckles
inward in a process called invagination.
• The buckled vegetal plate undergoes extensive
rearrangement of its cells, transforming the shallow
invagination into a primitive gut, or archenteron.
• The open end, the blastopore, will become the
anus. • An opening at the other end of the archenteron will
form the mouth of the digestive tube.
• Frog gastrulation produces a triploblastic embryo with an
• Where the gray crescent was located, invagination forms the
dorsal lip of the blastopore.
• Cells on the dorsal surface roll over the edge of the dorsal lip
and into the interior of the embryo, a process called involution.
• Once inside the embryo, these cells move away from the
blastopore and become organized into layers of endoderm and
mesoderm, with endoderm on the inside.
• As the process is completed, the lip of the blastopore encircles
a yolk plug.
• Gastrulation in the chick is similar to frog gastrulation in that it
involves cells moving from the surface of the embryo to an interior
• In birds, the inward movement of cells is affected by the large
mass of yolk.
• All the cells that will form the embryo come from the epiblast.
• During gastrulation, some epiblast cells move toward the
midline of the blastoderm then detach and move inward toward
• These cells produce a thickening called the primitive
streak, which runs along what will become the bird’s
• The primitive steak is the functional equivalent of the frog
• Some of the inward-moving epiblast cells displace
hypoblast cells and form the endoderm.
• Other epiblast cells move laterally into the
blastocoel, forming the mesoderm. • The epiblast cells that remain on the surface form
• The hypoblast is required for normal development and seems to help
direct the formation of the primitive streak.
• Some hypoblast cells later form portions of the yolk sac.
In organogenesis, the organs of the animal body form from the three
embryonic germ layers.
• Various regions of the three embryonic germ layers develop into the
rudiments of organs during the process of organogenesis.
• While gastrulation involves mass cell movements, organogenesis
involves more localized morphogenetic changes in tissue and cell
• The first organs to form in the frog are the neural tube and notochord.
• The notochord is formed from dorsal mesoderm that condenses
above the archenteron.
• Signals sent from the notochord to the overlying ectoderm cause that
region of notochord to become neural plate.
• This process is often seen in organogenesis: one germ layer
signaling another to determine the fate of the second layer.
• The neural plate curves inward, rolling itself into a neural tube that
runs along the anterior-posterior axis of the embryo.
• The neural tube becomes the brain and spinal cord.
• Unique to vertebrate embryos is a band of cells called the neural
crest, which develops along the border where the neural tube pinches
off from the ectoderm.
• Neural crest cells migrate throughout the embryo, forming many
• Some have proposed calling neural crest cells the “fourth germ
• Somites form in strips of mesoderm lateral to the notochord. • The somites are arranged serially on both sides along the
length of the notochord.
• Mesenchyme cells migrate from the somites to new locations.
• The notochord is the core around which the vertebrae form.
• Parts of the notochord persist into adulthood as the inner
portions of vertebral disks.
• Somite cells also form the muscles associated with the axial
• Lateral to the somites, the mesoderm splits into two layers that
form the lining of the coelom.
• As organogenesis progresses, morphogenesis and cell differentiation
refine the organs that form from the three germ layers.
• Embryonic development leads to an aquatic, herbivorous tadpole
larva, which later metamorphoses into a terrestrial, carnivorous adult
• The derivatives of the ectoderm germ layer include epidermis of skin
and its derivatives, epithelial lining of the mouth and rectum, cornea
and lens of the eyes, the nervous system, adrenal medulla, tooth
enamel, and the epithelium of the pineal and pituitary glands.
• The endoderm germ layer contributes to the epithelial linings of the
digestive tract (except the mouth and rectum), respiratory system,
pancreas, thyroid, parathyroids, thymus, urethra, urinary bladder, and
• Derivatives of the mesoderm germ layer are the notochord, the
skeletal and muscular systems, the circulatory and lymphatic
systems, the excretory system, the reproductive system (except germ
cells), the dermis of skin, the lining of the body cavity, and the adrenal
Amniote embryos develop in a fluid-filled sac within a shell or uterus.
• The amniote embryo is the solution to reproduction in a dry
environment. • The shelled eggs of birds and other reptiles, as well as monotreme
mammals, and the uterus of placental mammals provide an aqueous
environment for development.
• Within the shell or uterus, the embryos of these animals are
surrounded by fluid within a sac formed by a membrane called
• Reptiles (including birds) and mammals are thus amniotes.
• Amniote development includes the formation of four extraembryonic
membranes: yolk sac, amnion, chorion, and allantois.
• The cells of the yolk sac digest yolk, providing nutrients to the
• The amnion encloses the embryo in a fluid-filled amniotic sac
that protects the embryo from drying out.
• The chorion cushions the embryo against mechanical shocks
and works with the allantois to exchange gases between the
embryo and the surrounding air.
• The allantois functions as a disposal sac for uric acid and
functions with the chorion as a respiratory organ.
Mammalian development has some unique features.
• The eggs of most mammals ar